Magneto-optical memory elements have been actively studied as memory elements capable of recording, reading and erasing information. Particularly, the elements which employ a rare earth transition metal alloy film as a memory medium are very suitable, because the memory bits are not affected by grain boundary and the memory medium film can be made large.
In a magneto-optical recording and reading apparatus, polarized light is applied onto the magneto-optical memory element and the light which is reflected therefrom is subjected to the rotation of reflected polarized plane by magneto-optical effects, such as Kerr effect and Faraday effect, and is detected to read information.
FIG. 6 schematically shows the magnetic-optical recording and reading apparatus and FIG. 7 is a drawing explaining its functional principle.
In FIG. 6, 20 shows a semiconductor laser which generates linear polarized light 21 shows a collimator lens 22 is a polarizer 23 is a half mirror, and 24 is an objective lens. A analyzer 25 converts the polarized direction of the reflected light to light intensity. The number 26 is a photodiode which detects the output of the light intensity from the analyzer 25.
The light generated from the semiconductor laser 20 is preliminary changed through the collimator lens 21 to parallel light and then changed through the polarizer 22 to a first linear polarized light having a polarized direction of a in FIG. 7. The first linear polarized light a is converged through the half mirror 23 and the objective lens 24 onto a recording medium 28 formed on a transparent substrate 27. The first linear light a is reflected therefrom to form reflected light b or b' according to the magneto-optical effects (e.g. Kerr effect). The reflected light has a polarized direction (Kerr rotation angle of .theta..sub.k or .theta..sub.k') which corresponds with the recorded information of "0" or "1" stored on the recording medium 28 in the form of a magnetizing direction. For example, b corresponds to bit information "0" (an up magnetizing direction) and b' corresponds to bit information "1" (a down magnetizing direction). The reflected light is passed through the objective lens 23 and reflected by the half mirror 23 toward the analyzer 25. If the analyzer 25 is placed in the direction c of FIG. 7, it detects the light intensity d and d' which correspond to the polarized direction of the reflected light b and b'. Then, the photodiode 26 receives the reflected light b or b', which has an intensity of d or d', through the analyzer 25, and the information is read out as an electric signal corresponding to the intensity d or d' by a signal processing circuit (not shown in Drawings) connected to the photodiode 26.
As is apparent from the above mentioned explanation, in order to enhance the quality of readout signals, the photo-magnetic recording and reading apparatus in which reading of information is conducted by the Kerr effect of the magneto-optical memory element is required to have an increased Kerr rotational angle.
However, when the magneto-optical memory element comprises a rare earth transition metal alloy film as a memory medium, the Kerr rotation angle is small and insufficient to enhance the quality of readout signals.
In order to obviate the above mentioned problems, a Japanese Kokai Publication (unexamined) proposes a magneto-optical memory element which adopts a multi-layer construction. FIG. 8 shows a partial sectional view of the magneto-optical memory element of this construction.
In FIG. 8, 30 indicates a transparent substrate of glass, polycarbonate, epoxy resin and the like and 31 shows a first transparent dielectric film which has a higher refractive index than the transparent substrate 30. The number 32 is a rare earth transition metal alloy film 33 is a second transparent dielectric film, and 34 is a metal reflective film. In this construction, the rare earth transition metal alloy film is so thin that the light which reaches the alloy film partially passes therethrough. This construction has a Faraday effect which takes place upon passing the light through the rare earth transition metal alloy film 32, reflection from the metal reflective film 34 and again passing through the alloy film 32, in addition to Kerr effect which takes place by reflecting the light from the alloy film 32. Accordingly, the Kerr rotation angle appears to be increased several times, in comparison with the magneto-optical memory element only employing Kerr effect.
For example, in FIG. 8, where the transparent substrate 30 is glass, the first transparent dielectric film 31 is AlN, the rare earth transition metal alloy film 32 is GdTbFe, the second transparent dielectric film 33 is AlN and the metal reflective film is Al, the Kerr rotation angle appears to be increased to 1.6.degree.. On the other hand, the element which only employs Kerr effect has the Kerr rotation angle of about 0.3 to 0.4.
This construction, however, has the following defects.
(1) The memory element has a higher extinction ratio and is expensive, such as a Glan-Thompson prism should be employed as an analyzer.
(2) The element of the optical assembly increase in number, thereby increasing cost and increasing size.